The present invention relates to curable flame retardant compositions and more particularly to curable epoxy compositions containing a brominated triazine flame retardant additive which compositions are ideally suited for fabricating circuit boards.
Metal-clad boards, particularly such boards for use in fabricating printed circuits, are well-known in the art. The simplest of such boards generally comprises a resinous plastic (polymeric) substrate to which is bonded at least one thin sheet of an electrically conductive material, preferably copper. The resinous plastic substrate can be clad with the metal foil on one or both sides, depending upon the desired use, and can be rigid or flexible depending upon the composition of the resinous plastic substrate, the choice of reinforcement (if any), and the intended use of the board.
A number of polyphenylene ether compositions having favorable dielectric properties and utility in circuit board manufacture are known. However, there is a growing need in the industry for laminates with good thermal performance, solvent resistance, and improved dielectric properties such as dissipation factor and dielectric constant. Due to deficiencies in one or more properties, many such compositions have not attained wide commercial use. Specifically, while polyphenylene ethers are excellent dielectrics, deficiencies often are found in areas such as solvent resistance, flammability, and resistance to high temperatures. Polyphenylene ethers often are combined with polyepoxides in an attempt to improve upon the aforementioned properties, but such combinations have not been found to be wholly satisfactory either.
In addition to excellent dielectric properties, resinous compositions useful in printed circuit board manufacture also should be highly flame-retardant. A V-1 rating, as determined by Underwriters Laboratories test procedure UL-94, is universally required, with V-0 usually being necessary. The V-0 rating requires a flame-out time (FOT) of not more than 10 seconds in any trial and a cumulative FOT of not more than 50 seconds for five samples. As a practical matter, purchasers often mandate a maximum cumulative FOT of 35 seconds.
The fabricated board should not lose substantial weight and its surface should not be appreciably marred by contact with methylene chloride, a solvent commonly used for cleaning. Since connections with the printed circuit typically are made by soldering, the board must be solder-resistant as evidenced by the lowest possible percent increase in thickness (Z-axis expansion) when exposed to liquid solder at 288xc2x0 C. In addition to all these properties of the cured material, a relatively short curing time is highly desirable.
In preparing rigid metal-clad boards, it is common to form individual lamina, commonly called prepregs, by formulating a resinous binder composition made from epoxy, modified styrene, or the like. A solvent solution of the resin is placed in an apparatus known as a xe2x80x9cdip tank.xe2x80x9d Continuous webs of reinforcement can be pre-impregnated in the tank and then dried in a vertical or horizontal treating tower or oven. Normally, the resin is partially cured or B-staged after exiting the treater tower or oven. The copper foil, optionally coated with an adhesive, is placed on one side of the prepreg and subjected to heating under pressure to effect a bond between the metal foil and the substrate. Multiple prepregs can be used in forming a single composite board. Additionally, multilayer printed wiring boards will have a number of interposed laminae and copper sheets.
Pressing of the boards can be effected in a press by placing the foil/substrate structure between the platens and closing the press, or a continuous belt can be used. The curing cycle in the press will depend upon the nature and thickness of the laminate, the time and temperature of the cycle being those required to cure the substrate, and the bonding adhesive layer, if present. Sufficient pressure is required to effect adequate flow of the adhesive and/or substrate resins in order to wet-out and bond adequately. The pressure must be sufficient to prevent blistering which is due to the release of gases resulting either from retained volatiles in the substrate or adhesive layers, or resulting from by-products of the curing process.
Japanese patent 64[1988]-3223 describes blends of bisphenol-A diglycidyl ether, TBBPA diglycidyl ether, epoxy phenol novolac and curing agents such as amines. However, the resulting material produces laminates with inferior solvent resistance and is, thus, not suitable from printed circuit board laminates.
Japanese patents Hei 2[1990]-55721 and Hei 2[1990]-55722 describe laminates with improved chemical resistance, comprising (1) a bisphenol poly(glycidyl ether), epoxy novolac, and brominated bisphenol reaction product; (2) a poly(phenylene ether); (3) a novolac resin; (4) a lead salt; and (5) Sb2O3. However, the resulting trichloroethylene resistance still is not adequate in the cured product, and surface roughening is observed.
Walles et al. (U.S. Pat. No. 4,975,319) describe compositions comprising (1) a bisphenol poly(glycidyl ether), epoxy novolac, and brominated bisphenol reaction product; (2) a poly(phenylene ether); (3) a novolac resin, curing agents, and hardeners that produce laminates with dissipation factors in the range of 0.011-0.016.
Hallgren et al. (U.S. Pat. No. 5,043,367) describes (1) poly(phenylene ether); (2) halogenated bisphenol diglycidyl ether non-halogenated diglycidyl ether, and various curing agents which produce solvent resistant laminates having dissipation factors in the range of 0.011-0.013.
Katayose et al. (U.S. Pat. No. 5,218,030) describes the use of (i) a poly (phenylene ether) containing pendant allyl or propargyl groups; (ii) triallylcyanurate or triallylisocyanurate; and optionally (iii) a flame retardant; or (iv) an antimony-containing auxiliary flame retardant.
Katayose et al. (U.S. Pat. No. 5,352,745) disclose compositions with improved solvent resistance comprising a high molecular weight functionalized poly(phenylene ether) resin (xcex7=0.30-0.56 IV PPO) produced through reactive extrusion of poly (phenylene ether) with maleic anhydride. Formulation of (i) the reaction product of poly(phenylene ether) (PPE) with an unsaturated acid or acid anhydride with, (ii) triallylcyanurate or triallylisocyanurate, (iii) a brominated epoxy resin, (iv) novolac resins, and (v) a cure catalyst, produces flame retardant and solvent resistant resins useful in the production of printed circuit boards. This patent shows that a portion of the amino functionalized end groups contain capping residues. The skilled artisan would recognize that such end groups would comprise less than 10% of the hydroxyl end groups and would not be sufficient to significantly accelerate the cure rate of an allylic thermoset.
Chao et al. (U.S. Pat. No. 5,213,886) describe blends of low molecular weight poly(phenylene ether) compounds and epoxy resins. Tracy et al. (U.S. Pat. No. 5,834,565) describe blends of low molecular weight poly(phenylene ether) compounds in thermosetting matrices such as epoxy resins and unsaturated polyesters. These thermosetting compositions exhibit improved processability over analogous compositions containing high molecular weight poly(phenylene ether) compounds.
None of the foregoing art discloses the present flame-retardant blends that display excellent electrical properties, good solvent resistance, and good thermal expansion characteristics.
A curable composition is formulated from an epoxy resin; a flame retardant additive essentially free of phenolic groups and epoxy groups preferably having toluene solubility of greater than 15 g/100 ml toluene at 50xc2x0 C.; and a thermoplastic resin. The preferred epoxy resin component comprises an epoxy resin or mixture of epoxy resins with have on average greater than 2 glycidyl ether groups per molecule. The preferred flame-retardant additive is a brominated triazine compound. The preferred thermoplastic resin is a resin having a Tg greater than 130xc2x0 C. and most preferably a poly(phenylene ether).
The inventive curable compositions advantageously can be combined with reinforcement in the manufacture of circuit boards having excellent electrical properties, good solvent resistance, and thermal properties.